Stop-on-signal radio apparatus



Jan. 19, 1954 w. J. O'BRIEN 2,666,853

STOP-ON-SIGNAL. RADIO APPARATUS Filed June 5, 1950 5 Sheeis Sheet l 10 11 F. M. F. M. F. M.

R.F. AMP. CONVERTER |.F. AMP.

TUNING UNIT f A. M. A.M.

R.F. AMP. CONVERTER 20022707 01 m7fl/z/f a fa v/z ewd 5,

A.C. SOURCE Jan, 19,1954 w. J. OBRIEN STOP-ONSIGNAL RADIO APPARATUS Filed June 5, 1950 Jan. 19, 1954 w. J. OBRIEN 1 2,666,853

1 STOP-ON-SIGNAL RADIO APPARATUS Filed June 5, 1950 a :hets-s'fiei a & a

I o VOLTAGE Patented Jan. 19, 1954 STOP-ON-SIGNAL RADIO APPARATUS William J. OBrien, Oxshott, England, assignor,

by mesne assignments, to General Motors Corporation, Detroit, Mich, a corporation of Delaware Application June 5, 1950, Serial No. 166,233

23 Claims. 1

This invention relates to stop-on-signal radio apparatus for operation by amplitude modulated signals, frequency or phase modulated signals, or both. More particularly, it relates to stopon-signal apparatus, especially receivers requiring no manual presetting to tune in signals of predetermined strength when the signals are either amplitude or frequency modulated. Stopon-signal receivers have been also known in the art as stop-on-carrier, signal-seeking, or signaltuned receivers. Receivers of this general type have been described and claimed in previous patents, such as William J. OBrien Patent No. 2,426,580, issued August 26, 1947; also William J. OBr-ien, Patent No. 2,526,266, issued October 17, 1950, and Patent No. 2,569,426, issued August 25, 1951; and E. F. Andrews Patent No. 2,262,218; issued November 11, 1941, Patent No. 2,326,738, issued August 1'7, 1943, Patent No. 2,493,742,

issued January 10, 1950.

Although the invention to be described is especially adapted-for use with stop-on-signal receivers for broadcast reception, its use is not necessarily restricted to this field, as stop-onsignal apparatus may also be applied to radio transmitters, electronic controls, and other frequency selective apparatus.

In the past, stop-on-signal tuning apparatus has been generally directed toward the automatic tuning of amplitude modulated signals from broadcasting stations. The advent of frequency modulated broadcasting stations in greater numbers calls for extending the advantages of stop-on-signal tuning to the reception of frequency modulated signals either separate from or alternatively with amplitude modulated signals.

It is also advantageous to provide frequency modulated stop-on-signal means which operate in conjunctionwith amplitude modulated stopon-signal means in such a way that frequency modulated signals or amplitude modulated signals can be tuned in alternatively with the least complication and cost consistent-with satisfactory operation.

Certain components ordinarily used with frequency modulated signals may also be used with amplitude modulated signalsto facilitate a combined amplitude and frequency modulation de vice, and to take advantage of certain features of frequency modulated apparatus when receiving amplitude modulated signals.

An FM receiver employing a twin rectifier discriminator for detection is "correctly tuned at the .point .ofzzem average: voltage onzthe discriminator frequency voltage curve. With this arrangement, as the signal is approached the voltage rises to a maximum, falls to zero, and rises again to a maximum with opposite polarity. The correct zero point at the center of the characteristic 8-shaped frequency voltage curve is established when the rate of tuning is slow compared to the rate of signal frequency modulation so that the individual modulation swings are averaged out by delay in circuit response. The problem of operating stop-on-signal tuning so that stoppage is effected at zero or near zero average voltage is more difiicult than stopping it on or near a voltage frequency peak. If the first voltage peak of the 8 curve supplies the stopping signal, then the tuner must overrun the correct amount to the center of the curve. This requires that means be provided to compensate for varying signal intensity, and particularly for differences in tuner speed at the moment of stopping, as the overrun is a function of the tuning speed.

On the other hand, something more than a stepping signal in response to zero voltage is needed, because a stopping signal must not be produced by mere absence of a signal. To solve this problem, I may employ a device which tends to stop in response to a high voltage portion of the discriminator S curve, or in response to a high voltage portion of the voltage frequency curve of an associated circuit. However, this tendency is restrained or delayed until the dis criminator voltage arrives at or closely approaches zero. Thus the stopping signal depends on the close approach of one voltage to zero while another voltage exceeds a predetermined value. I have found that if the rate of tuning is not too fast, and if the near zero voltage con.

dition must persist for appreciable time before it permits the stopping signal to occur, satisfactory tuning of frequency modulated signals may be secured. Objectionable overrun of the tuning means may be prevented by causing the stopping signal to disconnect from the tuner all tuner operating parts having considerable momentum compared to the stopping friction. It has been found that by this or other similar methods objectionable overrun can be prevented, especially when the speed of the tuner is moderate.

In view of the foregoing, it will be understood that among the objects of my invention are the following:

To provide improved stop-on-signal radio apparatus and parts thereof; 1

. To provide stop onesignal radio apparatus adapted satisfactorily to tune in frequency modulated signals;

To provide radio apparatus capable of tuning in either AM or FM signals in the stop-on-signal manner;

To provide such an FlVL-AM or AM-FM stopon-signal receiver utilizing a twin rectifier discriminator for FM-AM or AM-FM stop-on-signal tuning;

To provide FM and/or AM stop-on-signal apparatus having means responsive to a predetermined voltage tending to cause stoppage and means for preventin the stopping signal until a restraining voltage falls substantially toward zero in response to the tuning in of either an FM or an AM signal.

It is a further object to provide means for producing a stoppage in response to the presence of a predetermined voltage and means for preventing stoppage until a voltage falls substantially toward zero.

It is a further object to provide a manual and automatic tuning system in which the operation of automatic startin means starts automatic tuning, and in which manual tuning may be performed without any other operation besides the tuning motion at all times except when the power tuning is in operation.

It is a further object to provide means discriminating against the production of a stopping signal in response to static or interference while tuning FM signals and/or while tuning AM signals.

A further object is to provide an AM-FM receiver in which the operation of a manual control will adapt the receiver to stop-on-signal FM tuning in one position or stop-on-signal AM tuning in another position.

A further object is to utilize an oscillator which changes its state responsive to the correct tuning in of a signal to provide the stopping signal for stop-on-signal tuning.

It is a further object to provide an oscillator which oscillates in response to an approach to zero voltage on its control element to tune in a signal which is correctly tuned when a voltage reaches or closely approaches zero.

It is a further object to provide a plurality of control means, and oscillator means, in which a first voltage tending to cause oscillation is connected to one control means, a second voltage which becomes zero at correct tuning is connected to another control means so as to prevent oscillation except in the vicinity of zero voltage, the oscillator means being coupled to the tuner stoppin means so that the tuner is stopped when the first voltage is above a predetermined value and the second voltage falls toward zero.

Further objects, advantages, and capabilities of my invention will become apparent upon study of the following description and accompanying drawings, in which:

Fig. 1 and Fig. 1a together constitute a schematic illustration of a complete receiver embodying my invention;

Fig. 2 is a partial wiring diagram showing a modification of part of the circuit shown in Fig. 1 and Fig. 1a; and

Fig. 3 is a graphical illustration of tuning curves, illustrating voltages obtained in certain circuits of the receiver in Fig. 1, shown plotted against frequency.

Fig. 1, together with Fig. 1a, represents a receiver of thesuperheterodyne type, suitable for the reception of either amplitude modulated or frequency modulated signals.

For more ready understanding of this invention, the general features will first be briefly described.

This receiver includes a tuner unit which may include a gang of FM tuning reactances operated by a single shaft. The receiver also includes frequency modulating receiving apparatus and amplitude modulation receiving apparatus. The tuner unit may be operated by a motor mechanism controlled by a stopping relay. This relay is in turn controlled by a stop-on-signal control device which may include a control and an oscillator tube. A common audio amplifier and speaker may be employed for both FM and AM reception, and also for amplifying the stopping signal from the control device, and supplying this signal to the stopping relay.

A manually operable switch is provided having an FM position and an AM position. In the FM position, the control device and the audio amplifier are connected to the FM receiving apparatus and the B voltage supply is connected to the FM receiving apparatus and disconnected from the AM receiving apparatus. When this switch is moved into the AM position, B voltage is supplied to the AM receiving apparatus and disconnected from the FM receiving apparatus, and the AM receiving apparatus is connected to the stop-on-signal tuning device and the audio amplifier. Thus the operation of this switch adapts the receiver for the stop-on-signal tuning of FM signals or AM signals, depending upon the osition in which this switch is placed.

The function of the stop-on-signal tuning device is, broadly, to cause the stopping relay to operate and stop the tuning unit when it tunes in a signal, but more specifically, its function is to cause the operation of the relay only when a desired signal is accurately tuned in. Correct tuning is indicated when the average voltage output of the discriminator falls from a peak of the S discriminator voltage versus frequency curve to zero voltage (as shown in curve Y of Fig. 3). However, zero voltage does not itself provide an operating signal. Such a signal may be provided by the peak voltage produced by a signal on the limiter grid (as shown in curve X in Fig. 3), or by the voltage rise occurring in one of the discriminator rectifier circuits which persists in each rectifier separately, considerably past the center of the discriminator output voltage frequency curve. During AM operation, the voltage may be the detector AVC voltage. These operating voltage sources do not by themselves provide a sufilciently exact indication of correct tuning. However, the combination of this actuating voltage applied to the control tube of the tuning device and a voltage from the discriminator applied to the oscillator tube, permit the latter to oscillate only when a negative voltage is applied to the control tube and when the voltage on the grid of the oscillator tube is reduced to the vicinity of zero. When the oscillator tube oscillates, its oscillatory voltage output is impressed upon the first audio tube which effects the operation of the stopping relay as this relay is coupled tothe output of the audio power amplifier during tuning. Thus the relay is operated when the tuning unit has tuned in either an FM or AM signal so that the discriminator voltage resulting from the signal has fallen substantially to zero. This occurs at the midpoint between the positive and negative discriminator voltage versus frequency peaks (as shown in curve Y, Fig. 3).

The motor mechanism includes a clutch which is operated by the stopping relay. The disen gagement of this clutch by the operation of the relay insures abrupt stoppage in response to the oscillation of the oscillator, so that little or no overrun occurs. For FM tuning the response of the circuits and the speed or the tuning unit may be suinciently slow with respect to the frequency of the modulation being received, so that oscillation of the oscillator occurs mainly in re sponse to the variation of frequency produced by the movement of the tuning unit, and without any major effect as a result of the variation in frequency due to modulation. stoppage of the tuner by static or other brief intense disturbances is also eliminated or greatly reduced by suitably delaying the start of oscillation.

Referring now in greater detail to Figs. 1 and 1a of the drawings, represents an antenna connected to the FM receiver. The frequency modulation receiving apparatus comprises a tuned FM radio frequency amplifier l I f and converter l2), tunable by their associated tuning means within a tuning unit I3. The converter I2 is coupled to the FM intermediate frequency amplifier l5).

The output of this amplifier is applied to the grid of the limiter tube I? by means of an intermediate frequency transformer 1%. Tube il may be a pentode, connected in well known manner, to function as an amplitude limiter. A resistor 89 is connected between the grid of tube l1 and ground. The cathode of tube 1! is grounded, and when a radio frequency voltage appears across the secondary of transformer l8, grid rectification produces a negative potential at the grid end of resistor I9. This direct current potential is applied to conductor 29. Capacitors 2| and 22 and resistor 23 provide radio frequency filtering.

The plate of tube H is connected to one end of primary winding 24 of discriminator transformer :25. The primary winding 24 may be tuned to resonate at center intermediate frequency by capacitor 25. The discriminator transformer 25 is of the well known type having a center tapped. secondary winding 21 inductively coupled to the primary winding 24, and tuned to resonance by means of capacitor 28. 'A capacitor 29 connects the plate end of primary winding 24 to the center tap 'of secondary winding 21. One end of secondary winding 2! connects to plate and the other end to plate 3| of a twin diode rectifier tube 32. The cathode 33, associated with plate 34, is connected directly to ground. Two resistors 34 and 35, of equal value, are connected in series between cathode 35 of tube 32 and ground. The center tap of secondary winding 21 is connected through radio frequency choke coil 31 to a point between resistors 34 and 35. The cathode 35 of tube 52 is bypassed to ground by a capacitor 38 and is connected to a contact 39 of the FM-Alvl changeover switch through a series resistor 4|. Plate voltage is supplied to FM units ll I2 and it as well as to the limiter tube [1, through conductor 42, which is connected to contact 43 of switch 40. Switch contact 44 of this switch is connected to conductor 28 which carries the negative potential developed in the grid circuit of limiter l'l when a signal is tuned in.

The amplitude modulation receiver apparatus which is not used for frequency modulation, com

s ts of an antenna I la; he R- F- m lifi Na.

and the converter 2a, with their associated .AM

variable tuning means in the tuning unit 13. The output of the converter lZa is applied to the grid of an intermediate frequency amplifier, which may be an electron tube 45 of the pentode type. The amplifier tube 45 is supplied with plate voltage through the plate winding 45 of the AM discriminator transformer 41, from conductor 48. which is connected to contact 45 of the change over switch 40, and which also supplies voltage to the units it. and 55 of the AM receiver section. Screen voltage for the tube 45 is obtained from a voltage divider consisting of two resistors 50 and 5 I, connected from conductor 45 to ground. Cathode bias for tube 45 is provided by resistor 52.

The plate winding 46 is closely coupled to the primary winding 53 of the discriminator transformer 4l. Primary winding 53 is tuned to resonate at the intermediate frequency by means of a capacitor 54. The secondary winding 55 is inductively coupled to the primary winding 53 and consists of a center tapped winding tuned to the intermediate frequency by a capacitor 55. One end of secondary winding 55 is connected to a plate 51 and the other end to a plate 55 of a twin diode rectifier tube 59. The cathode 85. of tube 59 is connected to the cathode 35 of the twin diode rectifier tube 32. The cathode E! of tube 59 is grounded. One end of primary winding 53 is connected to the center tap of the secondary winding 55. The other end of primary winding 53 is connected through resistors 52 and 63 to the junction between resistors 34 and 35. Due to the extra winding 48 closely coupled to the winding 53, the winding 53 is isolated from the voltage applied to the plate of the tube 45, and therefore no isolating condenser is required in the lead from the primary to the center tap of the secondary winding 55.

The junction point 54 between resistors 52 and 53 is connected to switch contact 55 of the change-over switch 40. An AVC resistor 55 is connected to junction point 64. Conduct-or 61, connected to the other end of resistor 56, is the AVG line which supplies automatic volume con,- trol voltage to the AM R. F. amplifier Ha, the converter l2a, and the intermediate irequency amplifier tube 45. Resistors 68 and 69, in series, are connected from point E ite ground and the junction between these resistors is connected through conductor 10 to a switch contact H of change-over switch 40.

The tuning means for both FM and AM reception may be operated by a rotatable shaft 12. A gang of variable condensers for FM tuning and another gang of variable condensers for AM tuning may have their rotor mounted on the shaft 12. If desired, variable inductances may be employed for tuning FM and/or AM. The tuner shaft '12 carries a lar e diameter pulley 13. The shaft 12 is driven by a cord 14 fastened to the pulley 13 and operating'on its periphery. For adequate frictional engagement, one or more turns of the cord 14 is wrapped around the small diameter shaft 15. The shaft 15 is rotatably mounted in bearing plates 16 and H, and is provided with a knob 18 for manual operation. A gear wheel 19 is secured to the shaft 15 and meshes with a pinion which is mounted on a shaft freely rotatable in bearing plates 16 and H. Pinion 80 is secured to a gear wheel 8| which meshes with a pinion 82, which is mounted on a shaft .83 for free .r tationin bearing plate 11.

gates The end of shaft 83 is operably engaged for free rotation by the armature 84 on the stop-on-signal relay 85.

Relay 85 is provided with two windings 06 and 81 around one leg of a ferromagnetic U member 88. The voltage drop across filter choke 89 is used to energize winding 86 when the tuning switch button 90 is operated. When this occurs, the armature 84 is moved against the tension of the armature spring 9I and closes the magnetic circuit across the open side of the U member 88. After switch button 90 is released, the armature 84 is held in closed position by the residual flux in the magnetic circuit of the relay until the bucking relay winding 81 is energized to produce a flux of a polarity opposite to that of the residual flux, thus permitting spring 9I to move armature 84 to open position. The means by which wind ing 81 is energized will be described more fully later. 7

A clutch member 92 is rigidly secured to the pinion 82 and is adapted to engage a clutch member 93, when the relay winding 85 is energized. This causes the armature 84 to move to closed position, thus moving shaft 83, pinion 82, and clutch member 92 to the left, as viewed in Fig. 1. Clutch member 93 is rigidly secured to a gear wheel 94 which is mounted on a shaft freely rotatable in bearing plate I6. Gear wheel 94 meshes with a pinion 95 secured to the rotor shaft of .a motor 96.

The motor 96 may be any type of reversible motor. However, I prefer to use an alternating current motor of the capacitor type having two similar field windings. A single pole double throw, maintained contact switch 91 is used to reverse the direction of rotation of the motor. This switch consists of a contact blade 98 and a contact blade 99 with a central contact blade I09 adapted to maintain a position to which it is moved on either side of center to complete the contact to either blade 98' or blade 99. This produces the desired direction of rotation of the motor. Switch 91 is operated by a double ended lever IOI connected for, rotation with shaft 12 of the tuning unit I3. The lever IOI is provided with switch operating members I02 and I03. These switch operating members, as shown, provide for 180 rotation of the shaft .12 beforethe direction of rotation is reversed by the switch 91. However, the switch operating members I02 and I 03 may be located to provide any desired degree of rotation that may be required to operate the tuning means contained within the tuning unit I3 through its full tuning range.

One side of the alternating current source of motor operating voltage is connected to ground through conductor I04, and the other side is connected through conductor I05 to the motor 96. Switch blade 98 of the reversing switch 9! is connected to one field winding of the motor by means of conductor I06. Conductor I 01 connects the other field winding of the motor to switch blade 99 of switch 91. The motor capacitor I 08 is connected between conductors I06 and I01.

A double pole double throw and single pole single throw switch I09 is operably connected to the armature 84 of relay 85. When the relay armature 84 is in the closed position, contact blade IIO, which is connected to ground, completes the circuit to the fixed contact III and through conductor I I2 to the center contact blade I00 of reversing switch 91. In this condition the motor field connected to conductor I06 is energized directly from the alternating current source I while the other motor field is energized from the alternating current source through the series capacitor I08; The closing of the relay armature moves the clutch member 92 into engagement with clutch member 93 to form a driving connection. The motor then drives shaft I5 through the three stages of reduction gearing. The rotation of shaft 15 is transmitted to the pulley I3 on shaft I2 by means of the cord I4, thus driving the switch operating lever IOI in a counterclockwise direction, as seen in Fig. 1. This motion continues, as long as the relay armature 84 remains in closed position, until switch operating member I02 engages switch blade 98 and moves it, together with blade I00, toward the switch blade 99. The central contact blade I00 is of the over-center snap type which snaps away from contact blade 98 and into engagement with contact blade 99. V

This operation of the switch 9'! causes a phase change in the field windings of the motor 96 to reverse its direction of rotation. The switch operating lever IOI then rotates in a clockwise direction until the switch operating member I03 engages contact blade 99 and actuates the central contact blade I00 into its original position in engagement with contact blade 98 to again reverse the direction of rotation of the motor.

The portion of the circuit below the FM-AM change-over switch 40 will now be described. Switch 40 is provided with three movable poles H3, H4, and H5. The switch 40 is shown in the FM receiving position with pole II3 completing a circuit to contact 39, pole I I 4 making electrical contact with switch contact 44, and pole H5 engaging switch contact 43. In this position of the switch, plate voltage is supplied to the PM section of the receiver through conductor 42, switch contact 43, and switch pole I I5.

When an FM signal is being received, the audio component of the frequency modulated signal appears at the cathode 36 of the twin diode rectifier tube 32. This audio frequency voltage is applied through resistor 4|, switch contact 39, switch pole H3, and capacitor III; to one end of a variable potentiometer II], the other end of which is connected to ground. The sliding contact II8 of the potentiometer I I! is connected through a resistor H9 and a coil winding I20 (to be later described), to the grid I2I of an electron tube I22. The tube I22 may be of the triode type having a cathode I23 and a plate I24. Cathode bias is provided for the tube I22 by the resistors I21 and I28 in series connected from cathode I23 to ground, and resistor I25, connected from the high voltage conductor I28 to the cathode .I23. In normal receiving position, the armature 84 of relay is open and the cathode resistor I28 is shorted to ground through the conductor I29 and the fixed contact I30 and movable blade I I0 of the relay operated switch I09. At this time only resistor I2"! is efiective to provide cathode bias for the tube I22. This provides the proper negative grid bias for the tube I 22 to function as an audio amplifier. Plate voltage is supplied to the plate I24 of tube I22 through the resistor I3I. The amplified audio frequency voltage developed across the resistor I3I may be coupled through a capacitor I32 to an audio frequency power amplifier I33.

The audio frequency power amplifier I33 may be of any well known type commonly used in radio receivers, and is not shown in detail. The amplifier I33 is provided with an output transformer I34 having a center tapped input winding I35, each end of which is connected to the output of the amplifier I33. This transformer has an output winding I36. The transformer shown is of the type required when the power amplifier I33 is equipped with a push-pull output stage. It should be understood, however, that an amplifier with a single ended output stage can also be used, and in that case, the center tap on the winding I35 of the output transformer I34 would not be required.

Plate voltage is supplied to the A. F. power amplifier I33 by means of a conductor I31 con necting the center tap of transformer winding I35 to the power supply filter choke 99. One end of the output winding I36 is connected to ground. The other end of winding I36 is connected, by means of conductor I38, to one end of the relay winding 81 and, by means of conductor $59, to one end of a voice coil I40 of a loud speaker MI. The other end of relay winding 81 is connected to a fixed contact I42 of the relay switch I09. The other end of the voice coil I40 is connected to a fixed contact I43 of the relay switch. A movable contact blade I44, operably connected to relay armature 84, is adapted to make electrical connection to fixed contact I42 in one position and fixed contact I43 in its other position. Blade I44 is connected to ground.

In the normal listening position, the movable contact blade I44 is in contact with the fixed contact I43. This completes the loud speaker circuit from ground through winding I35 of transformer I34, conductor I39, voice coil I40, fixed contact I43, movable contact blade I44, and back to ground. Thus the output of the audio power amplifier is supplied to the loud speaker and the volume level is determined by the position of the sliding contact II8 on the potentiometer II1. Signals may be tuned in manually by operation of the manual tuning knob 18, which drives the tuning unit shaft 12 through the drive cord 14. Operation of the tuning knob 18 also drives the pinion 82 in the gear train through the gear wheel 81, pinion 80, and gear wheel 19. HOW- ever, the motor shaft pinion 95 and the gear wheel 94 remain stationary since the clutch members 92 and 93 are disengaged except when the tuning unit is being driven by the motor.

For stop-on-signal operation, I provide a stop-on-signal tuning device employed for both AM and FM reception, comprising control tube I45 and oscillator tube I46, both of which may be of the pentode type. Plate voltage is supplied to the plate I41 of oscillator tube I45 through a series resistor I48 and a plate winding I49 of an oscillation transformer I50. This transformer is provided with a feedback winding II connected in series with the voltage supply to the screen grid I52 of the tube I46. Thus this oscillation transformer may be adapted to produce oscillations at a high frequency compared to the tuning speed, by feed-back from plate I4? to grid E52. An adjustable resistor I53 is connected from the cathode I54 of tube I46 to ground to adjust cathode bias and the grid voltage at which the tube breaks into oscillation. The grid I55 is connected by means of conductor I62 to cathode 35 of twin diode 32 and cathode 60 of twin diode 59 through a series resistor I63, and is bypassed to ground by a capacitor I64. Resistor I53 and capacitor I64 may be of such value as to provide a predetermined time constant. Thus the output of the FM discriminator double diode 32 applied to the grid I55 controls the start of'oscillation .10 of the oscillator I46 during FM operation and the output of the AM discriminator double diode 59 controls the start of oscillation of the oscillator I45 during AM operation. Voltage is sup-' plied to plate I56 of control tube I45 through the resistor I48 and a capacitor I51 is connected from plate I56 to ground to provide a predetermined time constant. :screen voltage is obtained for the screen H53 from a voltage divider circuit consisting of resistors I59, I60, and I53. Cathode I6I is connected to ground. The grid I55 is connected to pole H4 of the change-over switch 40. Thus the grid I55 is connected to the grid of limiter tube It during FM reception and to the AVG-detector voltage dividers 68 and 69 during AM reception. in this manner a negative voltage is impressed on the grid I65 in response to the tuning in of either an FM or AM signal. The grid I55 is also connected by means of conductor I66 to the fixed contact I61 of the relay operated switch I09. Contact I61 is engaged by the movable contact blade I68 to complete a circuit to ground when switch I09 is in normal receiving position. This is to prevent oscillation of the tube I46 during normal reception.

A capacitor I69 is connected from grid I65 to ground and may have a value in relation to resistance in the circuit, such as to provide a predetermined time constant for signals applied to grid I65.

At this point, attention may be called to discriminator output resistors 34 and 35, which function as the output resistors of the double diode 32 during FM operation, and as the output resistors for the double diode 59 during AM operation. By employing the same output resistors 34 and 35 for the FM and the AM discriminators, the grid I55 may be connected permanently to receive the output of either discriminator depending on which one is in operation, and no switching of the grid I55 from FM to AM circuits is required.

It will be noticed also that the movable switch pole H3 is connected through contact 39 and resistor M to the output of the FM discriminator during FM reception. This movable switch pole I i3 is connected through contact 65 and resistor 63 to the juncture between resistors 34 and 35 which are in turn connected to the AM discriminator acting as the AM detector. Thus the demodulated signal during both AM and FM reception is supplied to the volume control re sistor II'I through movable switch pole H3 and condenser H6. The alternating current detector output is impressed on the grid I2I of the first audio tube I22 through the volume control slider H8, isolating resistance H9, and oscillator transformer secondary I20. It may be pointed out that during stop-on-signal tuning the negative bias on grid I2 I, produced by the drop across cathode resistors I21 and I28, is sufficient to prevent any signal transmitted through the slider H8 from operating the stopping relay 85. However, operation of this relay is readily effected by the strong signal induced in the coil I20 when the tube I45 breaks into oscillation.

It will also be seen that the output of the audio amplifier is impressed on the bucking relay winding 31 when the armature 84 of this relay is "closed during the time motor 99 is varying tuning unit I3 to tune in a signal. After the signal is tuned inthe armature 8-4 is in the open magnetic circuit position, as shown in Fig. 1, and in this position the'voice coil I40 of the speaker is connected to 1L the audio output transformer for audible reception of either FM or AM signals.

The operation during the stop-on-signal tuning and reception of FM and AM signals will now be described. To receive FM signals picked up by antenna l), switch 40 is first placed in the right-hand position, as shown in Fig. 1a. This places the three movable poles H3, H4, and 5' of switch 40 in contact with stationary contacts 39, 44, and 43, respectively. This connects the FM discriminator output to the grid I2I of the first audio tube, applies B voltage to the FM receiving apparatus, and connects the grid of the stop-on-signal control tube I45 to the grid of the limiter tube I1. If the start tuning button 90 is now'pressed, the direct current voltage drop across choke coil 89 is impressed on winding 86. This attracts armature 84, engages driven clutch dogs 92 with driving clutch dogs 93, and at the same time contacts I I 0, I44, and I68 are moved to the left into engagement with contacts III, I42, and out of contact with contact I61 respectively.

The grounding of contact III starts motor 96 through connection II 2, switch blades I00 and 98, connection I06 and motor lead I05. The motor then drives the rotatable shaft 12 of the tuning unit I3 through large pulley 13, belt 14, small pulley 15, gear 19, pinion 80, gear 8I, pinion 82, gear 94, and pinion 95. If no FM signal of sufficient strength is tuned in by the tun'ng unit before the switch operating member I02 encounters the switch blade 98, the switch blade I60 will be moved over center and will snap out of engagement with contact 98 and into engagement with contact 99, thus reversing motor 96 and thereby the direction of rotation of the shaft 12 and the arm IIlI. If no signal of sulficient strength should be encountered, the switch operating member I03 will engage contact 99 and snap the switch blade I00 back to the position shown, again reversing the direction of tuner rotation.

However, upon encountering an FM signal of sufllcient strength, an increase in the voltage impressed on the grid of limiter I 1 takes place as the frequency of the output of converter I21 approaches the frequency at which the I. F. amplifier I6! is resonant. This voltage increase is illustrated by the curve X in Fig. 3. This I. F. signal is rectified by the grid of the tube I1, acting as a rectifier plate, and a direct voltage appears across resistor I9, connected between the grid and the cathode of the tube I1. The negative side of the resistor I9 is connected to the grid I65 of the tuning control tube I through the secondary of transformer I8, filter resistor 23, lead 20, contact 44, and movable switch pole I I4. As contact I68 has been moved away from contact I61, grid I65 is no longer grounded through lead I66. This negative voltage is thus applied to the grid I65 of the tube I45 substantially reducing or cutting off its plate current flowing from the B voltage source through conductor I26 and through resistor I48. The voltage drop across resistor I48 is thus reduced and the voltage on the plate I41 of oscillator tube I46 is increased, tending to cause tube I46 to break into oscillation. However, resistor I48 is so proportioned with relation to other circuit parameters that oscillation does not take place simply as a result of the increasing negative voltage, as shown by curve X, Fig. 3.

'Ihe bias on the grid of tube I46 has been previously adjusted, by means of variable resistor 153, so that oscillation will not occur in tube I46 as long as there is asubstantial negative bias on the grid. The curve Y of Fig. 3 represents the output of the PM or AM d scriminator plotted against frequency. The midpoint 0 indicates zero voltage which occurs at the center intermediate frequency, that is, the frequency to which the I. F. and discriminator transformers are tuned. This point 0 is the point of correct tuning for both FM and AM reception. However, for FM recept on, the point 0 must represent the average voltage over a period of time which is not short relative to the modulation frequency. This is so that the average modulation voltage will be zero and not merely the instantaneous modulation voltage. This, of course, does not apply in the case of AM reception. Voltages below the horizontal line are negative, and those above the line are positive direct current voltages. 7 7

It will be seen that when the voltage indicated by curve X has risen, for instance to a point A, a strong negative voltage represented by the point B on curve Y is impressed from the discriminator through delay resistor I63 on the grid I55, thus restraining or preventing oscillation. However, as the tuner brings the converter signal closer to the intermediate frequency, the voltage of curve Y falls from the point B toward the point 0 and at some point, for instance C, is sufficiently close to zero voltage so that the oscillator tube I46 is no longer prevented from oscillating by excess negative bias on its grid, and as it already has suflicient voltage on its plate I41, it breaks into oscillation. This produces a strong signal in the oscillator plate coil I49, inducing a corresponding voltage in the secondary I20 which is impressed upon the grid I2I. This causes a signal to be induced in the output transformer secondary I36, which is connected to the relay bucking coil 81 through movable relay contact I44, stationary contact I42, and conductor I38. The connections to coil 81 may be of such polarity that the first impulse will buck the residual flux in the U-shaped relay core or frame 88, which was set up by the direct current energization of the coil 86. This neutralizes or bucks out the residual flux and permits the spring 9| to move the relay armature 84 back to the position shown in Fig. l, at the same time moving the relay contact IIO, I44, and I68, and the driven clutch dog 92 back into the position shown in that figure. The variation of the tuning unit by the rotation of shaft 12 is thus abruptly terminated with the desired signal correctly tuned in. The motor circuit is opened by relay contact I I0. This contact is at the same time moved into engagement with stationary contact I 39. This shorts to ground resistor I28, in the cathode circuit of the first audio tube I22, thus reducing the negative grid bias on this tube to the proper value for good quality audio reception. Relay contact I68 is likewise moved into engagement with contact I61. This grounds the grid I65 of control tube I 45, thus removing the high negative bias therefrom. The increased plate current of this tube causes a greater voltage drop across resistor I48, reducing the voltage on the plate I41 of the tube I46 to a point which prevents the tube from oscillating, and preventing the oscillator from interfering with audio reception.

The FM signal will remain tuned in until the starting button is again pressed to repeat the operation of tuning in the nextFM signal, of sufficient strength; I I

' plotted amplitude versus frequency.

In the eventthat during the tuning-operation static or other brief interference is encountered, condenser Its with its associated resistors prevents a rapid build up of negative bias on the grid 55, thus reducing or preventing response to signals of short duration, Also, the condenser Il associated with resistor I48 tends to delay the change of voltage on the plate I41 and to delay oscillation.

Static and other like disturbances generallyconsist of broadly distributed high narrow peaks when In FM operation, these high peaks are largely cut off by the limiter which reduces the signal energy in the form of static more than it reduces the s gnal energy of a tuned signal. Tests have shown that this receiver can be made substantially unresponsive to heavy static and at the same time respending readily to relatively weak amplitude or frequency modulated broadcast signals.

The operation of the receiver for the stop-onsignal tuning of AM signals will now be described. It may be pointed out, however, that the receiver functions in the same general way for both FM, and AM, particularly with respect to the stopon-signal tuning device, audio amplifier, stopping relay and tuning unit drive, so that the description of these operations will not be repeated in detail. I

When it is desired to receive AM signals im pressed on the antenna Illa, the switch 4% is moved to the left, thus supplying B voltage to the AM receiving apparatus through the contact 45 and cutting off the voltage supply to the FM receiving apparatus through the contact 43. The connection to the grid of the l miter tube I! is cut oil and the grid IE5 is connected through movable pole H5, contact II, conductor It, and resistor 58, to point es, where the AM detector and AVC voltage appears. This voltage is reducedsomewhat by the voltage divider resistors 58 and 69. The grid E2I of the first audio tube I22 is dsconnected from the FM discriminator through the contact 39 and is connected to the output of the AM d scriminator or detector through contact 65, movable pole 5 I3, condenser H6, resistor IITI, slider H8, resistor Nd, and coupling coil I23.

t may be mentioned here that the resistor 63 improves the detector action of the AM discriminator as a detector, improving the audio quality. The detector output impressed on the grid of the first audio tube I22 appears across resstors 63 and 3 3 in series. That portion which appears across resistor as is due to the half wave rectification of the diode 58,1, while that portion across resistor 63 is due to the full wave rectification of diodes 58, iii, and 51, 50, as resistor 63 is in that portion of the circuit common to both diodes. It will be seen also that the AVG voltage which is supplied to the R. F. amplifier I la and the converter lza, also is taken from the point '64 through resistor 56 and conductor 61.

It will be seen that when the starting button as is pressed, and the tuning unit tunes in an AM station, the stop-on-signal action is substantially similar to that described for FM operation with certain exceptions. These include the fact that the negative voltage applied to the grid itfi, tending to cause the tube M5 to oscillate, supplied mainly by the diode 55, M, of the AM discriminator, and that only a part of the detector-voltage determined by the voltage divide ersiiii and 59 is employed. The output. of the AM discriminator applied to the gridI 55 restrain ing oscillation of this'tube' until the discriminator voltage approaches zerd'iS Similar to the action during FM operation, except that as there is no frequency variation as a result of modulation no time interval to secure an average voltage is required. However, resistor I63 and capacity I64 may remain in the circuit to avoid additional switching.

, The curves shown'in Fig. 3 are applicable to AM tuning as well as to FM tuning, except that the curve X shows the AM detector and AVC voltage instead of the limiter grid voltage as in FM operation. The frequency swing, or the width of the intermediate frequency pass band is also generally smaller for AM reception than for broad band FM reception.

At this point attention may be called to the fact that the operation up to this point has been described for the direction of tuning in which the converter frequency approaches the resonance frequency of the discriminator from the negative sideof the discriminator curve Y, that is, from the left side of Fig. 3. The action of the discriminator voltage on the grid of the oscillator tube I45 is somewhat different when tuning in the opposite direction, so that the positive peak of the discriminator curve Y is encountered first, that is, when'the converter frequency approaches the resonance frequency from the right as shown in Fig. 3. The application of the negative voltage to the grid I65 which tends to cause oscillation and which follows the curve X, in Fig. 3, is the same but on the opposite side of the curve at the point D. However, the voltage applied to the grid I55 indicated by the point E'on the curve Y is now a high positive voltage instead of a negative voltage. However, the parameters associated with'the tubes I45 and I 45 are such that the tubev IE5 also will not oscillate with a high positivevoltage on its grid. This is because the tube I46 draws a high plate current which increases the voltage drop across resistor Hi8 and reduces the voltage on the plate I41 to a point insufficient to produce oscillation. Therefore, the positive voltage on the grid I55 must fall along the curve ,Y to some point Fin the vicinity of zero voltage before oscillation can occur. It will therefore, be seen that oscillation is prevented by a predetermined positive or negative voltage on the grid I55 and oscillationcan occur only in the vicinity of zero voltage or, in other words, at the correcttuning frequency. This action is similar for both AM and FM.

Fig. 2 shows an alternate arrangement for FM operation in which the negative voltage applied to the grid H55 of the tube I45 is derived from the FM discriminator instead of from the limiter. Fig. 2 shows aportion of the FM receiving apparatus as shown in Fig. 111, except for modifications to be described. Corresponding parts are indicated by the same numbers.

It will be seen that the connection between contact ie of the FM.-AM conversion switch 48 has been replaced by the conductor I15, filter resistor Ill, connection i'i2, to a negative point with respect toground on the discriminator out-. put resistor as. Filter condenser I13 is connected between conductor lid and ground. It will be seen that the negative voltage tending to cause oscillation may be th'us supplied from the discriminator diode 3i, 36, if desired. Curve X serves to illustrate thevoltage appearing at the 7 point .i'iE as the correct "tuning frequency is,ap-.-

broached, The curVB Xwill not be identical in.

this case itofthatjof Fig'lq. but as the exact tunings; effected" in accordance-with the curve '15 Y, all that is required for proper operation is that curve X should provide a substantial negative voltage tending to produce oscillation at the center or zero voltage portion of the discriminator curve Y.

In this application I have explained the principles of my invention and the best mode in which I have contemplated applying those principles, so a to distinguish my invention from other inventions; and I have particularly pointed out and distinctly claimed the part, improvement, or combination which I claim as my invention or discovery.

While I have shown and described certain preferred embodiments of my invention, it will be understood that modifications and changes may be made without departing from the spirit and scope thereof, as will be clear to those skilled in the art.

I claim:

1. In stop-on-signal radio apparatus, a variable tuner means for varying said tuner, means for abruptly terminating variation of said tuner in response to a signal tuned by said tuner, including a discriminator network providing substantially zero voltage output when a signal is tuned in by said tuner, a relay operated quick stopping mechanism responsive to the tuning in of a signal by said tuner, and means coupled to the discriminator output effective to prevent operation of said mechanism until the discriminator voltage output closely approaches zero.

2. In stoponsignal radio apparatus, a variable tuner, means effecting variation of said tuner,

means for terminating said variation in response to the correct tuning of a signal by said tuner including tuner stopping means, means providing a first tuner responsive voltage which has an effectively high value when the signal is correctly tuned, means providing a second tuner responsive voltage which is approximately zero when a signal is correctly tuned in and which has a positive peak before correct tuning from one tuning direction and a negative peak before correct tuning from the other tuning direction, means placing said tuner stopping means in operable condition in response to the increase of said first voltage, and means responsive to said second voltage restraining said operation until said second voltage approaches zero.

3. In stop-on-signal radio apparatus, a variable tuner, means efiecting variation of said tuner, an amplifier, means including selective means pass ing a frequency band of sufficient width to transmitdesired modulation coupled to said tuner, means for abruptly terminating tuner variation in response to the tuning in of a signal by said tuner including quick tuner stopping means, a discriminator network producing output voltage of opposite polarity on opposite sides of a center frequency approximately coinciding with the center of said amplified frequency band coupled to said amplifier, means coupling said discriminator to said amplifier to apply an operative voltage to said quick stopping means as a signal is being tuned in by said tuner, and means controlled by said discriminator preventing operation of said quick stopping means except near the center of said amplifier frequency band.

' 4. In stop-on-signal radio apparatus, a variable tuner, means to vary said tuner, means responsive to a signal tuned in by said tuner for stopping tuner variation at said signal, including tuner stopping means, a discriminator network, the voltage output of which passes through zero '16 at the correct tuning frequency of said signal, and means coupling said discriminator output to said stopping means to effect tuner stoppage in the vicinity of zero discriminator output voltage.

5. In stop-on-signal radio apparatus, a variable tuner, means to vary said tuner, means responsive to a signal tuned in by said tuner for stopping tuner variation at said signal, including tuner stopping means, a discriminator network, the voltage output of which passes through zero at the correct tuning frequency of said signal, and means coupling said discriminator output to said stopping means including means to cause operation of said stop-ping means as the output voltage of said discriminator approaches zero.

6. In stop-on-signal radio apparatus, in combination, a variable tuner, means for varying said tuner, means for amplifying the output of said tune-r, including means passing a frequency band of suiiicient width to transmit modulation of desired quality, means for terminating variation of said tuner in response to a signal tuned by said tuner including tuner stopping means, a twin rectifier discriminator stop signal producing means connecting said amplifying means to said stopping means, said stop signal producing means being so constructed and arranged that it supplies no stopping signal in response to amplified frequencies at the outer edges of said frequency band while it supplies a stopping signal and stops said tuner in response to the central frequencies, of said band.

7. In frequency modulation radio apparatus, a variable tuner, means for varying said tuner, means for amplifying the output of said tuner, said means passing a frequency band of sufficient width to transmit frequency modulation of desired quality, a limiter coupled to said amplifying means, means for terminating variation of said tuner in response to a frequency modulated signal tuned by said tuner including tuner stopping means, a twin rectifier discriminator coupled to said limiter, each rectifier producing a voltage peak of opposite polarity to that of the other on opposite sides of the center frequency of said frequency band, and means controlled by said discriminator for effecting stoppage of said tuner at a point substantially equidistant with respect to frequency between the two voltage peaks, of said twin rectifiers.

8. In radio apparatus, a variable tuner, means for amplifying the output of said tuner, said means passing afrequency band of sufficient width to transmit modulation of desired quality. a limiter coupled to said amplifying means, means for terminating variation of said tuner in response to a signal tuned by said tuner including tuner stopping means, a twin rectifier discriminator coupled to said limiter, each rectifier producing a voltage peak of opposite polarity to that of the other on opposite sides of the center frequency of said frequency band, and means controlled by said discriminator for efi'ecting stoppage of said tuner at a point substantially equidistant with respect to frequency between the two voltage peaks of said twin rectifiers.

9. In signal controlled apparatus, a device to be operated, a normally blocked electronic oscillator having output elements and a plurality of control elements, means coupling said output elements to said device whereby said device is operated when said oscillator oscillates, means supplying a first voltage changing from a lower to a higher predetermined value connected to one of said control electrodes, means supplylnsa 17 second, voltage changeable from a: certain value of one polarity through zero toward a value of opposite polarity, means coupled to said oscilla-v tor causing oscillation when said first voltage reaches a certain value and preventing oscillation except when said second voltage approaches zero.

'10. In a stop-on-signal radio receiver for reception of amplitude and frequency modulated signals, a variable tuner, means for varying said tuner, means including selective means for amplifying and detecting the frequency modulated output of said tuner, means including selective means for amplifying and detecting the amplitude modulated output of said tuner, a low frequency amplifier and a reproducer, relay means for stopping the variation of said tuner in response to a signal tuned by the tuner, switch means controlled by said relay means effective to connect said relay means to said low frequency amplifier during variation of said tuner and to connect said reproducer to said low frequency amplifier when tuner variation is stopped, and manually operable switch means for connecting said low frequency amplifier to said frequency modulated amplifying and detecting means or to said amplitude modulated amplifying and de-' testing means to efiect the stop-on-signal tuning in of frequency modulated or amplitude modulated signals respectively.

ii. In a stop-on-signal radio receiver for reception of amplitude and frequency modulated signals, a variable tuner, means for varying said tuner, means including selective means for amplifying and detecting the frequency modulated output of said tuner, means including selective means for amplifying and detecting. the amplitude modulated output of said tuner, relay means for stopping the variation of said tuner in response to a frequency or amplitude modulated signal tuned by the tuner, means supplying a relay operating signal in response to the tuning of a frequency or amplitude modulated sig-- nal tuned by said tuner, means coupling said tuning signal means to said relay means, and manually operable switch means for connecting said tuning signal means to said frequency modulated amplifying and detecting means or to said amplitude modulated amplifying and detecting means to effect stop-on-signal tuning of frequency modulated or amplitude modulated signals respectively.

12. In a stop-on-signal radio receiver for recetion of amplitude and frequency modulated signals, a variable tuner, means for varying said tuner, means including selective means for amplifying and detecting the frequency modulated output of said tuner, means including selective means for amplifying and detecting the amplitude modulated output of said tuner, means for stopping the variation of said tuner in response to a frequency or amplitude modulated signal tuned by said tuner, and manually operable switch means for connecting said tuner variation stopping means to said frequency modulated amplifying and detecting means or to said amplitude modulated amplifying and detecting means to eifect stop-on-signal tuning of frequency modulated or amplitude modulated signals respectively.

3. In stop-on-signal radio apparatus, a variable tuner, means to vary said tuner, means for stopping said tuner, means including rectifying means supplied with a signal tuned in by said tuner and supplying a direct current voltage to operate said stopping means, twin rectifiers supplied with a signal tuned by said tuner and having output resistors connected in series with opposite polarity so that the voltage output of one is equal and opposite to the voltage output of the other at the correct tuning frequency of said signal, and means coupling said twin rectifiers to said stopping means to insure tuner stoppage at a position where the output of said twin rectifiers approach zero voltage.

14'. In stop-on-signal radio apparatus, a variable tuner, means to vary said tuner, means for stopping said tuner, means responsive to a signal tuned in by said tuner to operate said stopping means, twin rectifiers supplied with a signal tuned by said tuner and connected so that the voltage output of one is equal and opposite to the other at the correct tuning frequency of said signal, and means coupling said twin rectifiers to said stopping means to prevent tuner stoppage until the output of said twin rectifiers approaches zero voltage.

l5. Instop-on-signal radio apparatus, a variable tuner, means for varying said tuner, a discriminator network providing substantially zero voltage output when a signal is correctly tuned in by said tuner, a stopping mechanism responsive to the tuning in of a signal by said tuner, and means responsive to the discriminator output effective to prevent operation of said stopping mechanism until the discriminator voltage output closely approaches zero.

16. In stop-on-signal radio apparatus, in combination, a variable tuner, means for varying said tuner, means for amplifying the output of said tuner including means passing a frequency band of sufficient width to transmit modulation of desired quality, means for terminating variation of said tuner in response to a signal tuned by said tuner including, tuner stopping means, a twin rectifier discriminator stop signal networl: coupled to said amplifying means and to said stopping means, said stop signal network. being constructed and arranged to effect stoppage of said tuner only on the central frequencies of said amplified frequency band,

17. In stopon-signal frequency modulation apparatus, a variable tuner, means for varying said tuner, means for amplifying the output of said tuner, said amplifying means passing a frequency band only of sufficient width to transmit frequency modulation of desired quality, a limiter coupled to said amplifying means, means for terminating variation of said tuner in response to a frequency modulated signal tuned by said tuner including tuner stopping means, stop signal supply means coupled between said limiter and said stopping means, said signal supply means including means for preventing the stop signal from being supplied except in frequency band much narrower than and adjacent the center of said amplifying means frequency band.

18. In stop-on-signal radio apparatus, a variable tuner, means for varying said tuner, means for amplifying be output of said tuner, said plifying means passing a frequency band of cient width to transmit modulation of desired quality, a limiter coupled to said amplifying means, means for terminating variation of said tuner in response to a signal tuned by said tuner including tuner stopping means, stop signal sup ply means coupled between said limiter and said stopping means, said signal supply means includmeans for preventing the stop signal from being supplied except in a frequency band much narrower than and adjacent the center of said amlifying means frequency band.

19. In a stop-on-signal radio receiver for receiving tuned signals in the presence of brief intense interference, a variable tuner, means for varying said tuner, means for amplifying the output of said tuner, said amplifying means passing a frequency band of sufficient width to transmit modulation of desired quality, means for terminating variation of said tuner in response to a signal tuned by said tuner including tuner stopping means, stop signal supply means coupled between said amplifying means and said stopping means, said stop signal supply means including means for preventing; the stop signal from being supplied except in a frequency band much narrower than and adjacent the center of said amplifying means frequency band, and means including a limiter coupled between said amplifying means and said stopping means and circuit time constant means delaying the operation of said stopping means for preventing response to brief interference.

20. In a stop-on-signal radio receiver for receiving tuned signals in the presence of brief intense interference, a variable tuner, means for varying said tuner, means for amplifying the output of said tuner, said amplifying means passing a frequency band of suflicient width to transmit modulation of desired quality, means for terminating variation of said tuner in response to a signal tuned by said tuner including tuner stopping means, stop signal supply means coupled between said amplifying means and said stopping means, said stop signal supply means including means for preventing the stop signal from being supplied except in a frequency band much narrower than and adjacent the center of said amplifying means frequency band, and means including circuit time constant means for preventing response to brief interference.

21. In stop-on-signal radio apparatus, a, variable tuner, means for varying said tuner, means for terminating variation of said tuner in response to a signal tuned by said tuner including tuner stopping means, a twin rectifier discriminator coupled to said tuner, stop signal producing means coupling said discriminator to said stopping means, said stop signal producing means being so constructed and arranged as to prevent stoppage in response to the peak voltage output 20 of either discriminator rectifier while it permits stoppage of said tuner as the voltage output of one discriminator rectifier becomes equal and opposite to that of the other.

22. In a signal controlled apparatus, a device to be operated, an electronic oscillator, means coupling said oscillator to said device whereby said device is operated when said oscillator oscillates, a discriminator supplying a first control voltage to said oscillator, means supplying a second control voltage to said oscillator tending to cause oscillation when said second control voltage reaches a predetermined value, said oscillator being so constructed and connected that oscillation will commence only when said second control voltage reaches said predetermined value and when said first control voltage from said discriminator approaches zero.

23. In a stop-on-signal radio apparatus, a variable tuner, means for varying said. tuner, means for terminating variation of said tuner in response to a signal tuned by said tuner including tuner stopping means, a twin rectifier discriminator coupled to said tuner, stop signal producing means comprising an electronic oscillator, means coupling said oscillator to said tuner stopping means whereby said tuner stopping means is operated when said oscillator oscillates, a first control voltage supplied by said discriminator to said oscillator, means coupled to said tuner for supplying a second control voltage to said oscillator tending to cause oscillation as a signal is tuned in and when said second control voltage reaches a predetermined value, said oscillator being so constructed and connected that oscillation will commence only when said second control voltage reaches said predetermined value and when said first control voltage from said discriminator approaches zero.

WILLIAM J. OBRIEN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,262,218 Andrews Nov. 11, 1941 2,369,542 Dietrich Feb. 13, 1945 2,441,028 Mathews May 4, 1948 2,478,977 Nicholson Aug. 16, 1949 2,506,869 Gull May 9, 1950 2,529, 77 Case Nov. 14, 1950 

